The present invention relates to a semiconductor device, in particular, to a normally-off type MIS field effect transistor using a nitride semiconductor and a method of manufacturing the device.
Power semiconductor elements are used in power electronics semiconductor devices or high-frequency amplifiers. Power semiconductor elements are required to have low on resistance and high withstand voltage operation. To achieve such electrical properties, a nitride semiconductor having a greater band gap is used as an element material instead of conventionally used silicon. For example, Japanese Patent Laid-Open No. 2010-109086 discloses a nitride semiconductor element equipped with a first semiconductor layer of a p type nitride semiconductor, a second semiconductor layer of an undoped nitride semiconductor provided on the first semiconductor layer, a third semiconductor layer of an undoped or n-type nitride semiconductor provided selectively on the second semiconductor layer, a first main electrode provided on the third semiconductor layer, a second main electrode provided on the third semiconductor layer, an insulating film provided on the second semiconductor layer, and a control electrode provided on the insulating film, wherein the band gap of the third semiconductor layer is greater than that of the second semiconductor layer and the control electrode is located between the first main electrode and the second main electrode.
FIG. 1 is a cross-sectional view showing the structure of an MIS field effect transistor using a nitride disclosed in Japanese Patent Laid-Open No. 2010-109086. This MIS field effect transistor 440 is equipped with an underlying layer 401 which is a first semiconductor layer, an electron transit layer 402 which is a second semiconductor layer, an electron supply layer 403 which is a third semiconductor layer, a source electrode 404 which is a first main electrode, a drain electrode 405 which is a second main electrode, a gate insulating film 406, and a gate electrode 407 which is a control electrode.
The underlying layer 401 is a layer of p-GaN which is a p-doped nitride semiconductor. The electron transit layer 402 is a layer of i-GaN which is an un-doped nitride semiconductor formed over the underlying layer 401. The electron supply layer 403 is a layer of i-AlGaN which is an undoped nitride semiconductor formed over the electron transit layer 402. The electron supply layer 403 has thereover the source electrode 404 and the drain electrode 405. The source electrode 404 and the drain electrode 405 have therebetween a region which is free of the electron supply layer 403 and in this region, the gate insulating film 406 is formed and the gate electrode 407 is formed over the gate insulating film 406. This MIS field effect transistor 440 is characterized by that due to a piezoelectric effect and depolarization, a two-dimensional electron gas is formed at a hetero interface between the electron supply layer 403 and the electron transit layer 402, which leads to a reduction in on resistance. Since the gate region does not have the electron supply layer 403, a two-dimensional electron gas due to a piezoelectric effect and depolarization is not formed, making it possible to realize normally-off characteristics.                [Patent Document 1] Japanese Patent Laid-Open No. 2010-109086        